The invention relates to surfacing optical surfaces.
Surfacing means any operation aimed at modifying the surface state of a previously fashioned optical surface. This refers in particular to polishing, softening or depolishing operations aimed at modifying (reducing or increasing) the roughness of the optical surface and/or reducing undulation.
The invention relates to a tool for surfacing an optical surface, which tool comprises a rigid support having a transverse end surface, an elastically compressible interface that is pressed against and covers said end surface, and a flexible buffer adapted to be pressed against the optical surface and which is pressed against and covers at least part of the interface on the side opposite to and in line with said end surface.
To reduce the roughness of the optical surface, the tool is brought into contact with the latter and a sufficient pressure is maintained thereon for the buffer to espouse the shape of the optical surface as a result of deformation of the interface.
While spraying the optical surface with a fluid, it is driven in rotation relative to the tool (or vice-versa) and the tool is swept over it.
It is generally the optical surface that is driven in rotation, its friction against the tool being sufficient to entrain the latter in rotation conjointly with it.
The surfacing operation necessitates an abrasive, which may be contained either in the buffer or in the fluid.
During surfacing, the interface, which is elastically compressible, compensates the curvature difference between the end surface of the tool support and the optical surface so that the same tool is suitable for a range of optical surfaces with different curvatures and shapes.
If the transverse dimension of the tool is comparable to the dimension of the optical surface, which is generally the case when surfacing ophthalmic lenses, the range of optical surfaces that the same tool is capable of surfacing is relatively small.
This type of tool is particularly unsuitable for surfacing optical surfaces of complex shape, known as “freeform” surfaces, in particular aspherical surfaces, which by definition have a non-uniform curvature.
Furthermore, this type of tool is also unsuitable for optical surfaces having too marked a difference of convexity or concavity relative to the tool: in the former case, the edges of the tool lose contact with the optical surface; in the latter case it is the central portion of the tool that loses contact with the optical surface, as a result of which surfacing is incomplete.
There are two ways to enlarge the range of optical surfaces that the same tool is capable of surfacing.
A first is to reduce the diameter of the tool, i.e. its overall transverse dimension, so as to restrict and localize the portion of the optical surface in contact with the tool. The contact of the tool with the surface remains more homogeneous over a localized area of this kind than over the optical surface as a whole.
However, restricting the diameter of the tool reduces its “lift” or “seating” and therefore its stability on the optical surface during surfacing.
It is then necessary to monitor, and therefore to control, the orientation of the tool so that it is optimized at all times, i.e. so that the rotation axis of the tool is colinear or substantially colinear with the normal to the optical surface at the point of intersection of said axis with the optical surface.
Now this kind of control requires the use of complex means such as a numerically controlled machine, the cost of which is generally high and may even prove prohibitive for a surfacing operation.
A second option consists in retaining the same tool diameter but making the interface more flexible, either by increasing its thickness or by reducing its elasticity.
However, because of shear forces, the interface then tends to warp or to be offset laterally, to the detriment of the efficiency and accuracy of the tool. Furthermore, shear causes fast wear, or even destruction, of the interface. Finally, the flexibility of the interface encourages and accentuates the effects of the buffer scraping against the edge of the lens, which may eventually lead to the risk of premature and/or inopportune destruction of the tool.
Given the above, manufacturers of optical surfaces, and in particular manufacturers of ophthalmic lenses, have resigned themselves to having to use a large number of tools with different sizes and curvatures in order to cover the whole of their range of optical surfaces.